Vapor phase inhibitors of corrosion



Patented Mar. 3, 1953 VAPOR PHASE INHIBITORS. OF CORROSION Aaron Wachter' and Nathan"StillmanfBerlreley,

"Calif; assignors to Shell Development- Conrpany, San F-ran'cisco, CaIif.,-a corporation of Delaware No Drawing. Application June 1, 1946, Serial No. 673,886

The present invention relates to prevention of corrosion of metal objects which normally occurs in the presence of air (oxygen) and water vapor and/or aqueous condensates. More'particula-rly, this invention relates to amethod of inhibiting such corrosion of metal by providing an atmosphere containing one or more novel vapor-phase inhibitors in the immediate vicinity of metal objects, and/or by having immediate contact of the inhibitor with the metal. The invention also relates to novel compositions of matter formed by utilizing the present inhibitors with suitable carriers in certain embodiments of the present invention. A vapor-phase inhibitor maybe defined to comprise acompound which inhibits corrosion of metal because said compound possesses corrosion-inhibiting properties and is capable of vaporization under conditions of use with resultant presence of the vapors of the inhibitor in the vicinity of'the metal. A vapor-phase oxidative-corrosion inhibitor is a vaporephase inhibitor which inhibits corrosion of metal in the'presence of air (oxygen) and water vapor and/or liquid water. The metal may also be in partial contact with the unvolatilized. form :of the inhibitor. Our copending application, Serial No. 663,608, filed April 19, 1946, discloses and claims methods of inhibiting corrosion by the introduction of certain organicbase nitrites' as vapor phase inhibitors; the: present invention is particularly concerned with the introduction oforganic Neheterocyclic nitrites as vapor-phase inhibitors.

. During storage, handling, transportation, operation, and use of. objects having metal-containing surfaces such as tools, machines, moving metal parts, machined surfaces,;.sheets'of metal, steel cans, tanks, metal containers, shell-cases, fire-arms, rocket-heads, metal tubes, pipes, and conduit-enclosures, it .isoften necessaryto prevent the corrosion of these metals caused by the inevitable or practically unavoidable presence of water vapor and air (oxygen). In the past, protection against: such corrosion and concomitant loss of the use of metal were not satisfactory. In many cases it was conventional to cover the metal surfaces with a heavy oil or a grease which then" had to' be removed before the metal parts could be used as desired. The application and removal of such a protective coating or others was difficult, time consuming, and wasteful of labor and wealth. Such a' method as well as various other methods tried heretofore to prevent the afore-mentioned corrosion were unsatisfactory because .such methods did not adequately 18 Claims. (CI. 21-25) -2 prevent the corrosion, were cumbersome, and/or required an excessive amount of labor and time.

It is, therefore, an object of the present invention to provide vapor-phase inhibitors'which prevent corrosion of metal objects in presence of watervapor and oxygen (as in air). A further object is'to provide vapor-phase inhibitors which prevent such'corrosion of metal present inside relatively= impermeable, highly impermeable, or completely impermeable containeror conduitwalls, or of ametal disposedunder a covering material. It is also an object of this invention to provide vaporizable corrosion inhibitors, which, in the substantial vicinity of a-metal, prevent corrosion of the surfaces of various corrodible metals. It is a further object of this invention to provide volatile corrosion inhibitors, and methods-and means of applying same whereby an atmosphere containing said corrosion inhibitor vapors is provided so that. corrosion of surfaces of metal in contact therewith and water vapor'is significantly inhibited or even prevented. It is still another object of this invention to provideamethod for preventing corrosion of metal surfaces during storage, shipping, use, and the like. One ofthe primaryobjects of theinvention is toprovide corrosion inhibitors andmeans -of utilizing same whereby'an atmosphere substantiallysaturated with corrosion inhibitor vapors is maintained around a metal object normally subject to corrosion when in the: presence of air and moisture. Other objects, together with advantages derived. therefrom, will become apparent from the following description of the present invention.

It has 'now been discovered that corrosion of the surfaces of a corrodible'metal in the presence of Water vapor and oxygen (as in-air) is prevented by' the presence'of the-vapors of-a salt of'nitrous acid and an N-heterocyclic organic base-preferably by contact with vapors of one or more salts formed by the addition of nitrous acid (HNOz) to atrivalent basic nitrogen atom of an N heterocyclic organic amine. More particularly, it was found that such corrosion is prevented by anatmosphere, or other contacting material, containing vapors of thesaidN-heterocyclicorganic base nitrite salts'wherein dispersion of such 'an atmosphere or contacting material-in water forms an aqeous phase having apI-I value of at leastabout 5, and preferably abovea pH of about 6. Preferably, the vapors of the nitritesalt of an N heterocyclic organic amine 'which may be, for example, morpholinium nitric or piperidinium nitrite, should be at least in part water-soluble, although the degree of water-solubility may be very low. In some cases a barely detectable or incipient solution of vapors of an N-heterocyclic nitrite formed in an aque ous condensate on a metal surface may be sufiicient to prevent corrosion. It is not necessary for visible aqueous condensates to be present, since the present invention also operates satisfactorily in the presence of water vapor without visible aqueous condensates present on metal surfaces. It was found that the present vaporphase inhibitors preferably should have a vapor pressure of at least 0.00002 mm. Hg at 21 C. Better results are obtained with inhibitors having a vapor pressure greater than about 0.00001 mm. Hg at 21 C. More rapid inhibition of corrosion is obtainable with a vapor phase inhibitor having a vapor pressure greater than about 0.001 mm. Hg at 21 C.

The term N-heterocyclic organic amine, as used herein, refers to a trivalent basic N-heterocyclic nitrogen atom-containing compound, that is, a true N-heterocyclic organic amine, and not to a pentavalent basic N-heterocyclic quaternary ammonium compound.

The N-heterocyclic base which is reacted with nitrous acid, preferably in the presence of at least a slight excess of the base for the purpose of preparing the substantially pure nitric salt, is a base which desirably has a basic dissociation constant at least approxmately as high as the acidic dissociation constant of nitrous acid. Preferably the basic dissociation constant of the N-heterocyclic base is higher than the acidic dissociation constant of nitrous acid. For example, morpholine has a basic dissociation constant greater than Kb=4 10- at 18 C. as compared to the acidic dissociation constant of nitrous acid, viz. Ka=4 10- at 18 0.

Most preferably a secondary N-heterocyclic organic amine is reacted with nitrous acid to form a secondary N-heterocyclic organic amine nitrite salt for the purposes of the present invention. The less preferred tertiary N-heterocyclic organic amine nitrite salts may also be used. Quaternary ammonium N-heterocyclic base nitric salts may also be used as vapor phase corrosion inhibitors, but, in general, they are somewhat less economically practical or efficacious that the above-mentioned secondary or tertiary amine nitrites.

A suitable vapor phase inhibitor consisting of or comprising an N-heterocyclic organic amine nitrite salt (which may be briefly expressed as an N -heterocyclic nitrite) may also be prepared from nitrous acid and an N-heterocyclic amine, the hydrocarbon radical substituents of which are different, or an N-heterocyclic amine containing a plurality of amino groups in the heterocycle, or additionally on a side chain. A compound employed as an N-heterocyclic nitric vapor phase inhibitor according to the present invention may contain an acyclic, alicyclic, or other heterocyclic structure. The hydrogen atom of the basic nitrogen atom of the secondary N-heterocyclic amine, and/or one or more hydrogen atoms attached to the heterocyclic ring may be substituted .with other hydrocarbon radicals which may be heterocyclic, aromatic, acyclic, or alicyclic radicals. The substituents on the basic nitrogen atom may have aliphatic unsaturation or aromatic unsaturation or both. Nitrous acid may add to the aliphatic unsaturation or otherwise react with the organic basic structure in addition to adding to the basic nitrogen atom so long as the nitrite salt "e11 the N-heterocyclic basic nitrogen atom is 4 formed. The organic substituents forming part of the N-heterocyclic structure or attached thereto may contain stable polar radicals, e. g., chloro, fluoro, bromo-ether, thio-ether, alcohol, free amino, ketone, ester, nitrite, cyanate, nitrile, or nitro groups. The ring containing the nitrogen atom (n) may be N-cycloaliphatic or may contain atoms other than carbon such as one or more oxygen, sulfur, or other atoms which do not interfere with the basic properties of the ring-nitrogen atom and its formation of nitrite salts. The N- heterocyclic ring may be substituted with alkyl, alkenyl, naphthenic, cycloparaflinic, cyclo-olefinic, aralkyl, aromatic, and/or heterocyclic radicals.

Some illustrative examples of the above types of suitable substituent organic radicals are:

Alkyl groups: for example, C'4I-I9--, CsH13-, CsH17, 0111-123, C12H25-, C1sI-I3'1, etc.

Alkenyl groups: for example, allyl, methallyl, crotyl, oleyl, pentenyl, hexenyl; octenyl, isobutenyl, etc.

Cycloalkyl or naphthenic: for example, cyclohexyl, alkylated cycloliexyl, methylcyclopentyl,

cyclopentyl, mixtures of naphthenic groups, e. g.,

those obtanable from petroleum, etc.

Cyclo-olefinic: for example, cyclohexene, methyl cyclohexene, 'alkylated cyclopentenes, etc.

Aralkyl: for example, benzyl, alkylated benzyl, etc.

Aromatic radicals: for example, phenyl, naphthyl, diphenyl, alkylated aryl groups, etc.

Heterocyclic radicals: for example, oxazine, thiazoline, pyrazole, piperazine, piperidine, pyrrolidine, thiophane, furane, pyridine, thiophene quinoline, etc.

Terpinyl radicals: for example, bornyl, fenchyl, etc.

The organic cyclic structure, connected to the intra-cyclic basic nitrogen atom, may be either saturated or unsaturated. Any of the nuclear carbon atoms may be substituted with a suitable group as described above and hereinafter. The more preferred substituents are alkyl groups having less than twenty carbon atoms.

Examples of suitable N-heterocyclic nitrites applicable to the practice of the present invention include:

Such N-heterocycloaliphatic nitrites as the nitrite salts of pyrrolidine, alkylated pyrrolidines,

piperidine, alkylated piperidines, hydro-(ruinolines, piperazine, alkylated piperazines, imidazoles, etc.

Such N-heterocyclic nitrites as the nitric salts of oxazines, dioxazines, preferably hydro-oxazines, thiazolines, preferably hydrothiazolines, thio-oxazines, pyrazoles', diazoles, imidazolines, alkylated forms of these heterocyclic amines, for example, 2,4,4,6-tetramethyl tetrahydro -l,3- oxazine, 2,4,4,6-tetramethyl tetrahydro l,3- thiazoline, also diazines, preferably hydro-diacines, and similar compounds.

In the oxazines and suitably substituted oxazines, the oxygen atom and the amino nitrogen atom forming part of the organic ring, may be in any position relative to each other provided that there is at least one carbon atom intermediate between them. Such a structure insures that the ring nitrogen atom retains its basic ability and forms the required nitrite salts. In the case of preferred six-membered heterocyclic rings, an oxazine may be either a 1,3 or a 1,4 oxazine.

Preferred oxazine nitrites are alkylated or unalkylated l/l-oxazine nitrites, e. g., morpholinium nitrite, or 1,3-oxazine nitrites, e. g. 2,4,4,6-tetramethyl tetrahydro-L3-oxazine nitrite.

N.-heterocycloaromatic nitrites such as those derived from pyridine, picoline, 'quinoline, nicotine, and the C-alkylated and/or Cvcycloalkylated pyridines, picolines, quinolines, and the like are also applicable to the practice of the present invention. They are somewhat less preferable than the non-aromatic-N-heterocyclic type of nitrite salts.

Quaternary N-heterocyclic ammonium base salts of nitrous acid suitable to the practice of the present invention include the following nitrite salts: N -dipropyl pyrrolidine nitrite, piperidine N-diethyl nitrite, pyridine N-ethyl nitrite, pyridinium N-lauryl nitrite, piperazine N-dimethyl nitrite, quinolinium N -benzyl nitrite, and various other nitrite salts of pyridinium and alkylated pyridinium or quinolinium quaternary ammonium base compounds with or without suitable substituents on the ring (s) and having an alkyl, cycloalkyl, or aralkyl group on the quaternary nitrogen atom, including methyl, isoproyl, butyl, cyclohexyl, benzyl groups and like homologs or analogs.

The N-heterocyclic nitrites applicable in the present invention may be prepared by a variety of methods. These organic nitrites may be prepared, for example, by stoichiometric reaction between an N-heterocyclic amine and nitrous acid carried out in preferably an, at least slightly, alkaline media. Alternatively, such N-heterocyclic nitrites may be prepared by reacting the salt of an inorganic acid, preferably the hydrochloride or sulfate of an N -heterocyclic amine with sodium nitrite in a media in which the resultant N-heterocyclic nitrite is relatively insoluble compared to the sodium salt produced by double decomposition. Thus, a salt, e. g., sodium chloride is left dissolved in the media whereas the N-heterocyclic nitrite precipitates.

Compositions which are employed for the purposes of the present invention consist of or comprise, in general, the nitrite salts of organic heterocyclic nitrogen bases which contain oneor more amino nitrogen atoms as connecting members within one or more such organic heterocycles, and which may or may not contain one or more hetero atoms, preferably an oxygen atom, less preferably a sulfur atom, as members of the heterocyclic structure. For the sake of economical practicality and convenience, usually a nitrite salt of a, compound containing one heterocyclic nitrogen base nucleus is preferred. Polynitrite salts of a compound containing two or more heterocyclic nitrogen base nuclei are also useful for the purposes of the present invention. The ring structure containing the one or more basic nitrogen atoms preferably should contain at least five atoms, a six-membered ring being the most preferred structure. The present nitrite salts may be expressed as having a general formula which contains the following essential nucleus:

wherein D represents the atoms, preferably carbon atoms, necessary to complete an organic heterocyclic nitrogen compound, and the two R's are selected from the group consisting preferably of the hydrogen atoms, or from among such substitutents as the hydrogen atom, an alkyl, aralkyl, alkarylalkyl, alkenyl, aralkenyl,

alicyclic, terpenic, orheterocyclic radical. anyot which. may containsubstituent groups or elements. As entities, the present .N-heterocyclic nitrites are chemically distinct from.N-heterocyclic nitrosamines.

Although the proportion oramountof a present vapor phase inhibitor which is maintained in the vicinity or partial contact with a metal part to prevent corrosion thereof may varywithin wide limits, it is preferred to maintain approximately the smallest effective amount. It will be appreciatedthatthe required amount of a nitrite salt of an N-heterocyclic basic nitrogen atom employed as a vapor phase inhibitor varies depending on a number of variables, e. g., the severity of conditions, temperature, degree of humidity, etc., under which it is employed, also the particular N-heterooyclic nitrite applied, and the mode in which it is applied. Usually the placing of a sheet of material, paper, or an absorbent material impregnated with or coated with the organic nitrite (of the defined class) in the proximity of the metal enclosed in a package or other housing or enclosure means, provides satisfactory inhibition of corrosion where between about 0.1 gm. and about 5.0 gm., or preferably approximately between 1.0 gm. and 2.0 gms. of the inhibitor is present per square foot of the inner container surface. More particularly, satisfactory results are attained when the. inhibitor is present in such an amount as to allow between about 1 gm. and about 15 gm. (for average conditions about 6 gm.) thereof for each cubic foot of enclosed vapor space.

Stability of the present -N-heterocyclic basic nitrogen atom nitrites is adversely affected by an environment which on contact with or dispersion in water, yields a solution having a pH of less than about 6. In some instances the stability is also adversely afiected by elevated temperatures, e. g., F. or F. Such and like factors which may render any particular one or more of the specified group of N-heterocyclicnitrites unstable, are greatly lessened or entirely obviated by the presence of an alkaline or basic agent according to our copending patent application Serial No. 668,016, filed May 7, 1946, now

abondoned. An alkaline agent is a compound which reacts as a base in the presence of acidicreacting material andthereby neutralizes the acidic properties of the latter.

In some cases a selected specific N-heterocyclic amine nitrite may be in equilibrium with corresponding nitrosamine and water which are formed from the nitrite, and the shift of the equilibrium toward the nitrosamine is often increased at increasingly higher temperatures. This equilibrium may be stated as follows:

Heat

wherein R NI-I represents a N-heterocyclic amine structure or wherein the R portion of the organic ring contains suchhetero atoms as oxygen and/or sulfur, and/or more nitrogen atoms. According to the usual laws of equilibrium, an excess of either water or nitrosamine or both above the chemically equivalent amounts indicated in the above equation tends to force the reaction toward increasing the concentration of the N- heterocyclic amine nitrite. The presence of the nitrosamine is not substantially detrimental to vapor phase inhibition of corrosion; in fact, vapors of the nitrosamine itself probably-by the above-reaction with water,-are foundto inhibit the corrosion of metal in the presence of water vapor. However, in a number of cases, such vapors do not appear to be as effective for corrosion-inhibiting as vapors of the corresponding amine nitrite salt when the latter is substantially free of nitrosamine.

The present vapor phase inhibitors comprising salts of nitrous acid and N-heterocyclic nitrogen bases can be applied either separately or in various mixtures of specific different N-heterocyclic nitrites, or in mixtures with other vapor phase inhibitors, for example, those disclosed in our copending atent application Serial No. 663,608, filed April 19, 1946.

The vapors of the present salts of nitrous acid and N-heterocyclic nitrogen bases are particularly effective in preventing corrosion of ferrous metals, e. g. steels, as well as aluminum, nickel, chromium, and of alloys of these metals.

For purposes of illustration, reference will be had to the following specific examples, it being understood that there is no intention of being limited to the specific conditions disclosed.

EXAIVIPLE I Vapor pressure measurements on N-heterocyclic amine nitrites Vapor pressure Compound: in mm. X Piperidinium nitrite 0. 57 Morpholinium nitrite 2,4,4,G-tetramethyl-tetrahydr 1 These vapor pressures were measured by the dynamic method,

applying an air flow rate of 0.007 cubic feet per hour.

EXAMPLEII A thoroughly cleaned steel specimen was Wrapped in kraft paper and then placed in an external metal foil paper envelope. In a duplicate envelope, a duplicate steel specimen was wrapped in the same kraft paper but impregnated with 0.1 gram of morpholinium nitrite. The external metal foil paper envelopes were sealed with a hot iron, and the resultant packages were then totally immersed in synthetic sea water maintained at a temperature of approximately 30 C. for a period of 25 days. in the package without morpholinium nitrite was badly rusted, having rust generally distributed over the entire surface of the steel, but the steel in the package containing the kraft paper impregnated with morpholinium nitrite was discovered to have retained its original polished and unblemished appearance. In fact the morpholinium nitrite protected metal had no trace of corrosion.

EXAMPLE III A sample of piperazinium nitrite was tested as a vapor phaseinhibitor of the corrosion of steel. The tests were made by suspending a 4 inch by 2 inch low carbon steel specimen from a fresh clean cork used to stopper a 4-ounce sample bottle. One bottle contained upon its inner bottom surface, one gram of piperazinium nitrite and one gram of distilled water. The control bottle for the test contained only distilled water, air, and the steel specimen above the water. The steel specimens were not in contact with the walls, bottom, or materials contained within the sample bottles. Just before testing, the steel specimen was cleaned thoroughly by brushing vigorouslywith a steel wire brush, then washed with acetone The steel I and dried. The two sample bottles with their experimental assemblies were tested by maintaining them at a temperature of 50 C. for 24 hours except that once during this time, they were chilled to 12 C. for one hour, then reheated to 50 C. and then maintained at this temperature. The changes in temperature were designed to simulate adverse conditions encountered in practice of storing, shipping, or using steel in industry. The control test made simultaneously with that on the specified EN-heterocyclic nitrite showed that the control specimen corroded in less than one hour after starting the initial heating. On the other hand, the steel specimen subjected to the vapors of the N-heterocyclic nitrite was completely protected from any noticeable corrosion during 24 hours of operation. This steel had retained its original polished and unblemished appearance, whereas the steel in the control test was very badly rusted.

EXAMPLE; IV

EXAMPLE V One-half by 2 inch specimens of steel were suspended in the air-space above 0.5 gm. of 2,2,4,6-tetramethyl-piperidinium nitrite and 0.5 ml. of water contained in a 4-ounce glass-stoppered bottle which was then maintained at approximately F. for 7 days. The metal thus subjected to the vapors of the piperidinium nitrite was not corroded and had only a slight tarnished surface appearance, no rust being evi dent. In a control test, the steel was very badly rusted.

The present application is a continuation-inpart of the copending application Serial No.

557,358, filed October 5, 1944, now abandoned.

We claim as our invention:

1. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the steps of disposing said metal in an enclosed space, and disposing, in said space, a minor amount sufficient 'to produce a corrosion-inhibiting concentration of the vapors thereof of morpholine nitrite.

2. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of maintaining, in contact with said metal, an atmosphere containing a corrosion-inhibiting concentration of vapors of morpholine nitrite, said at mosphere, upon dispersion in water yielding a solution having a pH value of at least about 6.

3. In a method of inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors 0f morpholine nitrite, said atmosphere, upon dispersion in water yielding a solution having a pH value of at least about 6.

4. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the step of disposing, said metal in an enclosed space, and disposing in said space; a minor amount sufficient to produce a corrosion-inhibiting. concentration of the vapors thereof of 2,2,4,6-tetramethyl-piperidinium nitrite.

5. In a method for inhibiting corrosion of a ferrug inous metal normally corrodible by. contact with water vapor and oxygen, the step of maintainingin contact with said metal, an atmosphere containing a corrosion-inhibiting concentration of vapors of 2,2, 1,6-tetramethylpiperidinium nitrite.

6. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the step of forming, in the immediate vicinity of the metal, and subjecting said metal to an atmosphere containing a corrosion-inhibiting concentration of vapors of piperazinium nitrite.

7. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the step of forming, in the immediate vicinity of said metal, and subjecting said metal to an atmosphere containing a corrosion-inhibiting concentration of vapors of a nitrite salt of an N-heterocyclic organic amine, said heterocyclic amine having at least 5 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least about 0.0001 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least about 6.

8. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with vapor and oxygen, the step of forming, in the immediate vicinity of said metal, and subjecting said metal to an atmosphere containing a corrosion-inhibiting concentration of vapors of a nitrite salt of an N-heterocyclic organic base, said heterocyclic base having at least 5 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least 0.00002 mm. Hg at 21 C., said atmosphere, upon dispersion in Water, yielding a solution having a pH value of at least about 6.

9. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the step of forming, in the immediate vicinity of said metal, and subjecting said metal to an atmosphere containing a corrosion-inhibiting concentration of vapors of 2,4,4,6-tetramethyl-tetrahydro-3-oxazine nitrite.

10. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors of the nitrite salt of an N-heterocyclic organic amine, said heterocyclic amine having at least 5 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least 0.00002 mm. Hg at 21 C., said atmosphere, upon dispersion in Water, yielding a solution having a pH value of at least about 5.

11. A method according to claim 1 wherein morpholine nitrite is present in an amount between 1 and 15 grams per cubic foot of the enclosed space.

12. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere contain- 10 ing a corrosion-inhibiting concentration of vapors of the nitrite salt of a tetrahydro 1,4: ox'azine, said nitrite salt having a vapor pressure of at least 0.00002 mm.Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least about 5.

13. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors of the nitrite salt of a secondary N-heterocyclic organic amine, said heterocyclic amine having at least 5 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least 0.00002 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least about 5.

14. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors of the nitrite salt of a tetrahydro N- heterocychc azine, said azine containing no atoms other than C, H, O, and N, with any 0 and N atoms being in the heterocyclic ring and separated by at least one O atom, said nitrite salt having a vapor pressure of at least 0.00002 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least about 5 15. In a method for inhibiting corrosion of a ferrous metal normally corrodible by contact with water vapor and oxygen, the step of forming, in the immediate vicinity of said metal, and subjecting said metal to an atmosphere containing a corrosion-inhibiting concentration of vapors of a nitrite salt of an N -heterocyclic organic amine, said heterocyclic amine having from 5 to 6 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least about 0.00001 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least about 6.

16. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors of a nitrite salt of N-heterocyclic organic base, said heterocychc base having at least 5 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least 0.00002 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least 5.

17. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors of a nitrite salt of N-heterocyclic organic base, said heterocyclic base having at least 5 atoms in the ring structure thereof, said nitrite salt having a vapor pressure of at least 0.0001 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yieldin a solution having a pH value of at least 6.

18. In a method for inhibiting corrosion of a ferruginous metal normally corrodible by contact with water vapor and oxygen, the step of contacting said metal with an atmosphere containing a corrosion-inhibiting concentration of vapors of the nitrite salt of an alkylatecl piper- 1 1 idine, said nitrite salt havlnga vapor pressure of at least 0.00002 mm. Hg at 21 C., said atmosphere, upon dispersion in water, yielding a solution having a pH value of at least 5.

AARON WACHTER. NATHAN STILLMAN.

REFERENCES CITED Number 12 UNITED STATES PATENTS Name Date Cox Apr. 5, 1933 Bottoms Dec. 22, 1936 Cook Apr. 29, 1941 Parker Dec. 15, 1942 Briggmann July 6, 1943 Fuller et a1 Aug. 17, 1943 Giloy Mar. 14, 1944 Smith Sept. 5, 1944 Herlocker et a1 Oct. 30, 1945 Wachter Oct. 11, 1949 

8. IN A METHOD OF INHIBITING CORROSION OF A FERROUS METAL NORMALLY CORRODIBLE BY CONTACT WITH VAPOR AND OXYGEN, THE STEP OF FORMING, IN THE IMMEDIATE VICINITY OF SAID METAL, AND SUBJECTING SAID METAL TO AN ATMOSPHERE CONTAINING A CORROSION-INHIBITING CONCENTRATION OF VAPORS OF A NITRITE SALT OF AN N-HETEROCYCLIC ORGANIC BASE, SAID HETEROCYCLIC BASE HAVING AT LEAST 5 ATOMS IN THE RING STRUCTURE THEREOF, SAID NITRITE SALT HAVING A VAPOR PRESSURE OF AT LEAST 0.00002 MM. HG AT 21*C., SAID ATMOSPHERE, UPON DISPERSION IN WATER, YIELDING A SOLUTION HAVING A PH VALUE OF AT LEAST ABOUT
 6. 